1. Field of the Invention
The present invention relates to a frequency synthesizer, and more particularly, it relates to a frequency synthesizer in which a reference signal to be internally used can be stabilized over the entire period of service immediately after power activation.
2. Description of the Related Art
In a frequency synthesizer used in a base station of the next generation mobile communication, ground digital broadcasting or the like, the improvement of accuracy with respect to a reference signal or the improvement of start characteristics has been desired.
Examples of a reference signal source include a cesium (Cs) standard oscillator, a rubidium (Rb) standard oscillator, a frequency synchronous type standard oscillator by GPS signal, an oven controlled crystal oscillator (OCXO) and a temperature compensated crystal oscillator (TCXO), and these sources have different frequency accuracies.
Therefore, the reference signal source which satisfies an accuracy necessary for each use application is utilized.
The OCXO is a crystal oscillator comprising a constant temperature oven which maintains a temperature, and the TCXO is a crystal oscillator which beforehand stores, in a memory, temperature compensation data to compensate for fluctuations due to a temperature, thereby correcting the temperature.
[Target of Frequency Stability of Oscillator: FIG. 6]
The frequency stabilities of oscillators used as reference signal sources will be described with reference to FIG. 6. FIG. 6 is a schematic explanatory view showing the frequency stabilities of the oscillators used as the reference signal sources.
As shown in FIG. 6, in general, the TCXO has a frequency stability (Δf/f) of about 10−6, the OCXO has a frequency stability of about 10−8, and the rubidium oscillator and cesium oscillator utilizing the resonance phenomenon of atoms satisfactorily have frequency stabilities of about 10−10 and 10−12, respectively.
However, the rubidium oscillator and cesium oscillator are expensive, and hence it is not realistic to mount the oscillators in mass products.
Moreover, the frequency of the OCXO noticeably fluctuates for several minutes until the temperature of the constant temperature oven reaches a fixed temperature. The TCXO only requires a short time until it stabilizes, and hence, has satisfactory rising characteristics, but the frequency stability of the oscillator for a long period of time is poor as compared with the OCXO.
On the other hand, it is known that the phase noise characteristics of the OCXO are more satisfactory than those of the TCXO.